I have the Spark scroll bar in my application skinned the way I want where I use s|VScrollBar and then set skinClass: ClassReference(my skin class) in my CSS file. For my Halo scroll bar I have the skins set for each element of the scroll bar (arrows, thumb, track, etc.) but I want to have one central class like with the Spark scroll bar. Is there any way to do this?
Yes, but not in the same way as you can in Spark. Spark was made for easy skinning which is the great thing about Flex 4, but Flex 3 did things very differently.
What you can do is extend the Flex 3 scrollbar into a custom class that sets all those elements in it. Then within that project, just reference that custom class. Sadly, that's all that can be done.